You ask what subgenre hasn’t been exploited? The post-mortem photography one! In fact, I believe the filmmakers behind the upcoming supernatural horror flick The Mourning Portrait might be the first to focus an entire horror film around it.
Back in the early days of photography, when people died, their families would pay to have photos taken of their dearly departed as a means to preserve their memories of being together. Ghoulish -- yes. Hard to look at -- sometimes. Perfect fodder for a horror movie -- DEFINITELY.